Piezoelectric ceramics are generally composed of an ABO3 perovskite-type metal oxide such as lead zirconate titanate (hereinafter referred to as “PZT”). However, since PZT contains lead as an A-site element, the effect of PZT on environment is regarded as a problem. Therefore, piezoelectric ceramics composed of a lead-free perovskite-type metal oxide have been required.
An example of the piezoelectric ceramics composed of a lead-free perovskite-type metal oxide is an alkali niobate compound (general formula: MNbO3, M is an alkali metal). Perovskite-type metal oxides containing sodium niobate, such as a solid solution of sodium niobate and barium titanate or a solid solution of sodium niobate and potassium niobate, are promising as alternative compounds for PZT. Such an alkali niobate compound has an advantage in that sintering can be performed at a low temperature of, for example, 1300° C. or less.
A metal oxide powder serving as a raw material for obtaining a piezoelectric ceramic from the alkali niobate compound is generally produced by a so-called solid state reaction including a step of mechanically mixing or kneading each metal oxide, a step of molding the mixture into a pellet, and a step of firing the pellet.
In recent years, a synthesis method in a liquid phase has also been considered. For example, PTL 1 discloses a synthesis method that uses a solvothermal process. This synthesis method provides alkali niobate particles that have a shape of a cuboid and a uniform size and are suitable for piezoelectric ceramics.